Yes, BAEC Silver can use the lounge in BKI when flying economy on MH or KA.
https://www.oneworld.com/ffp/lounge-...ungeaccess/BKI
Malaysia Airlines Golden Lounge
Location details: Airside, Level 3 right after Gate 7
Opening hours: During flight operations
https://www.oneworld.com/ffp/sapphire-privileges
When your top tier status is equivalent to oneworld Sapphire, you are entitled to the following oneworld benefits, no matter which oneworld airline or cabin class you are flying:
Access to priority Business Class check-in
Priority boarding
Access to preferred or pre-reserved seating (where offered)
Priority standby and waitlisting (where offered)
Access to more than 600 airport lounges (with one guest) when travelling internationally, including premium Business Class and frequent flyer lounges, regardless of the class of service flown that day. For details about the oneworld lounge access policy, click here.